The present invention is directed to a wrapping machine for wrapping a tray.. having fresh food or the like placed thereon with a stretchable film. More particularly, this invention relates to a wrapping machine which folds a wrapping film covering the top of a tray under the tray to wrap it.
One conventional wrapping machine is disclosed in, for example, Japanese laid open patent No. 52-132984. This wrapping machine is equipped with a film feeder which feeds a film drawn from a film roller of a film roller support to a wrapping portion. This wrapping machine also has a material lift device, which carries up or down a tray received below the wrapping portion and causes the film in the wrapping portion to abut on the tray to lift the film with the upward/downward elevation, and a material conveyor device which feeds a tray onto a material lift of this lift device.
Further, the prior art wrapping machine is provided with a film folding device, which enables a rear folder to move closer to and away from a front folder secured at the wrapping portion and side folding arms to open and close with the movement of the rear folder thereby to fold the film under the tray. The wrapping machine further comprises a heater located next to the front folder of the film folding device on the transfer side, and a material transfer device located above the film folding device.
When the film is folded against the tray by the individual folders and side folding arms in the wrapping machine, the edges of the film are held to permit the film to be stretched under the tray by the folders and side folding arms and to be folded there while applied with tension. In this case, insufficient tension is liable to wrinkle the film and overstretching the film is likely to tear the film. It is therefore important to adjust the tension of the film to the tray in order to provide an excellent wrapping state.
Since the film is made of a synthetic resin, its elongation varies with changes in the atmospheric temperature. The tension of the film to the tray is therefore greatly affected by such change in the atmospheric temperature, making it difficult to keep a good wrapping state.
The film feeder of the wrapping machine disclosed in the aforementioned Japanese laid open Patent No. 52-132984 has a fixed film holder for holding the leading edge of the film fed out from the film roller and a movable film holder to move closer to or away from the fixed film holder to hold the leading edge of the film on the fixed film holder. In the vicinity of this fixed film holder is a support bar rotatably supported, with a pressing plate attached to the upper end of the support bar. The film is drawn from the fixed film holder by the movable film holder, and the film in this state is held by the pressing plate near the fixed film holder.
According to the above-described film feeder, however, the support bar with the pressing plate attached thereto rotates to hold the film, thus requiring a link mechanism to cause the rotation. This makes the wrapping machine large-scaled due to the space needed for the link mechanism as well as complicates the overall structure.
The movable film holder disclosed in the aforementioned publication has an openable and closable holding portion which is opened or closed by a drive section directly secured to the movable film holder.
Since the drive section moves integrally with the movable film holder, however, another device cannot be provided on the moving locus of the drive section in order to avoid interference with the drive section. It has therefore not been possible to efficiently utilize the space above the moving locus. Thus the wrapping machine must be relatively large, which is disadvantageous.
According to the above prior art wrapping machine, it is not possible to detect if the film is actually drawn from the film roller by a predetermined length, nor is it possible to detect if the drawn film is actually supplied to the wrapping portion by the film feeder. Even when the desired length of the film is not supplied to the wrapping portion, the film folding motions will continue. In such a case, the tray may not have been wrapped with the film or the wrapping may have been done incompletely.
The film roller support used in the conventional wrapping machine is exemplified in FIGS. 15 and 16. This support has a pair of supporting rollers 32 and 33 rotatably provided in parallel to support a film roller 34, so that a film 35 drawn from the film roller 34 is drawn upward, passing between the supporting rollers 32 and 33 or around the supporting roller 32, as indicated by the one-dot chain line in FIG. 15.
The leading edge of this film 35 is held by a fixed film holder 43. When the leading edge of the film 35 is pulled toward a wrapping portion by the fixed film holder 43, the film roller 34 rotates in the direction of the arrow 121 and both the supporting rollers 32 and 33 rotate in the direction of the arrow 122.
Even if drawing of the film 35 from the fixed film holder 43 is completed, however, the film roller 34 and supporting rollers 32 and 33 may rotate due to the inertia. In this case, a mid portion 35a of the film 35 may be caught between the film roller 34 and the supporting roller 33 as indicated by the two-dot chain line in FIG. 15 or a mid portion 35b of the film 35 wound around the supporting roller 32 may be caught between the film roller 34 and supporting roller 32 as shown in Fig. 16, possibly hindering the drawing of the film To provide a good wrapping state, the above wrapping machine requires that the length of a film in use and the timings for holding, releasing and/or cutting the film be adjusted in accordance with the size of a material. Such adjustments are premised that the size of the material should accurately be detected.
The width of a material along the conveying direction of the material can be computed by a product of the OFF time of a photoelectric switch, which is switched off while the material is passing it, and the material transfer velocity, as disclosed in Japanese laid open Patent No. 62-122917, for example.
The length of a material along a direction orthogonal to the conveying direction of the material may be computed by providing a pair of guide bars whose interval can be adjusted in accordance with the size of a material and which guide both sides of the material during transfer, and detecting the interval between both guide bars by means of a potentiometer, as disclosed in Japanese laid open Patent No. 62-193910, for example. The use of this detection means, however, requires adjustment of the interval between the guide bars for each material, making the interval adjustment originating from alteration of the material troublesome.
Alternatively, providing a plurality of photoelectric switches in parallel on the line orthogonal to the conveying direction of a material is proposed as disclosed in Japanese laid open Patent No. 63-149510. Although this detection means can automatically detect the length of a material based on the ON/OFF status of each photoelectric switch, its structure is inevitably complicated due to the necessity of a number of photoelectric switches.
A material feeder of a wrapping machine of the same type as the wrapping machine disclosed in the aforementioned Japanese laid open Patent No. 52-132984 is described in Japanese laid open Patent No. 59-124226, for example. According to the conveyor of the material feeder, placing a material on a material support activates the overall wrapping machine and rotates a conveying belt having a circular cross section (hereinafter called "round belt") located below the material support. When a lift moves down to a material receiving position at one end of the belt, a round belt mounting frame moves above the material support to cause the belt to contact the material and the material is conveyed onto the lift by the belt. When the material is separated off the material support, the round belt mounting frame moves back downward and the belt is positioned below the material support.
According to the above conveyor, the belt moves upward together with the belt mounting frame with the downward movement of the lift, thereby conveying a material. As long as an operator places a material accurately on the material support, therefore, the material is conveyed in synchronism with the elevation timing of the lift to accurately place the material on the lift. If the operator erroneously places a material partially protruding from the material support, the material may contact the rotating belt to be conveyed regardless of the elevation timing of the lift, and is liable to hit the lift.
According to the conventional conveyor, as described above, while the timing for placing a material on the support is in no way restricted, the material may be carelessly conveyed due to negligence of an operator.
The wrapping machine disclosed in the aforementioned Japanese laid open Patent No. 52-132984 is provided with a lift which moves upward and downward with a material received below a wrapping portion, and a conveyor for supplying a material onto the lift. When the lift having a material placed thereon moves upward, the material abuts on a film supplied to the wrapping portion, from below, and the material is covered with the film from above. This film is folded under the material, and the wrapped material is then transferred.
Since a mechanism for elevating the lift upward and downward is linked to the bottom of the lift under the conveyor, however, it is necessary to provide space for this elevating mechanism below the conveyor. This inevitably requires that the position of the conveyor and the lowest elevating position of the lift be set high, thus enlarging the overall wrapping machine.
According to the wrapping machine disclosed in the aforementioned Japanese laid open Patent No. 52-132984, every time a single tray is supplied, a wrapping machine drive signal is outputted to activate the wrapping machine. When the wrapping machine is intermittently activated by continuous supply of trays, the first tray supplied is transferred when the third tray is supplied. When there is no tray supplied, the drive signal is not outputted to deactivate the wrapping machine, leaving trays unwrapped in the wrapping machine. In this case, there requires a troublesome operation to press a manual switch to activate the wrapping machine to transfer the trays left therein.